The present invention relates to an annular support for rolling bearings.
As known, rolling bearings are mechanical devices in which a relative motion, for instance between a rotating member and a stationary member (or between a pair of members rotating at different speeds), takes places with the interposition of balls or rollers rolling between two tracks, one of which is directly formed on the rotating member or a ring integral therewith, and the other is formed on the stationary member or a second ring integral therewith. The balls or rollers are generally spaced by a variously shaped cage, capable of separating and holding the balls or rollers.
In some industrial applications, a resilient support is to be interposed between the bearings and the seat housing them, or between the bearings and the rotating member, e.g. a shaft. The resilient support is to compensate an alignment error, if any, of the same bearings and to prevent vibrations, generated also due to the alignment error, from propagating from the shaft to the structure of the machine in which the shaft is mounted.
Another source of shaft vibrations is a non-uniform mass distribution about the geometrical rotation axis, resulting in centrifugal forces in the rotor and hence in the bearings thereof. One of the applications where use of resilient support is generally provided for concerns rotary vacuum pumps, more particularly turbomolecular vacuum pumps of the kind equipped with mechanical bearings.
As known, rotary vacuum pumps are equipped with rotating shafts supported either by magnetic bearings or by mechanical rolling bearings, in which the shaft rotates at extremely high speed, typically in the range 20,000 to 90,000 rpm.
When the pump is equipped with mechanical bearings, in order to prevent vibrations of the shaft or the pumping rotor associated therewith from propagating to the pump structure, annular supports of elastomeric material are provided, which surround the rolling bearings.
FIG. 1 shows an example of turbomolecular pump in which a rotor 101 is equipped with rotor discs 103 that, by co-operating with stationary stator discs integral with the pump housing (not shown), provide for gas pumping between the inlet port and the discharge port of the pump.
The rotor 101 is mounted onto a rotating shaft 105 supported by ball bearings 107a, 107b. The rotating shaft 105 is rotated by an electric motor 109 housed within a cavity 111 formed in pump basement 117.
As clearly shown in the enlarged portion of FIG. 1, a resilient annular support 113, made of one or more rings, is provided between each ball bearing 107a, 107b and the corresponding seat 115 formed in pump basement 117.
The provision of annular supports 113 is due above all to the need to damp vibrations transmitted by the rotating portions of the pump to the body of the same pump and, through the pump, to the vacuum chamber.
Actually, in some particularly critical applications (such as in mass spectrometry), where the vacuum pump is used in association with very sophisticated measurement instruments, it is indispensable to prevent the vibrations of the pump rotor from being transmitted to the remaining structure and in particular to the instruments.
A direct consequence of the damping of these vibrations is moreover the reduction of the overall pump noise.
The presence of annular supports 113 also contributes to a considerable reduction of the first critical speed (intended as the first rotation speed associated with a modal form with substantially non-deformed rotor—“rigid rotor”—and at which the force transmitted to the bearings has a maximum), which in this manner is far lower than the nominal pump rotation speed, with a resulting effect of rotor self-balancing when critical speed is exceeded.
Further, the presence of such annular supports allows compensating any alignment error depending on the mechanical working of the bearing seats: said alignment errors are sometimes considerable and exceed the limits recommended by the manufacturers for high-speed precision bearings, used for instance in turbomolecular pumps.
According to the prior art, said annular supports are resilient and they are preferably made of an elastomeric material, such as nitrile rubber. Yet, using elastomeric supports entails a number of drawbacks.
For example, in these supports the mechanical characteristics of the material are dispersed, it is practically impossible therefore to attain uniform mechanical while different ring points exhibit different rigidity. For the same reason, it is infeasible to manufacture several rings of elastomeric material all exhibiting the same mechanical properties as well to predict their behaviour at the design phase.
Moreover, the properties of the elastomeric material become degraded in time, due to the applied loads and the operating temperature (often exceeding 60° C.), with resulting loss of resiliency and permanent deformation of the support. Such loss of resiliency has, among other drawbacks, severe consequences on the balancing of the rotor that, being no longer correctly held in radial direction, transmits stronger vibrations to the pump body.
It is also to be considered that, for the kind of use described above, elastomeric rings would require considerable geometric precision (in respect of diameter sizes, diameter concentricity etc.), which is extremely difficult to attain with such materials.
Further, it should be taken into account that ball bearings mounted in vacuum pumps are generally submitted to an axial preload. For instance, in the case shown in FIG. 1, the preload is exerted by spring 119. The considerable axial friction existing between the outer ring of the ball bearing and the elastomeric annular support may significantly hinder the proper application of this preload, especially as concerns bearing 107 a farther from the preload spring 119 arranged in the pump basement.
In order to by-pass the latter problem, it has been proposed to introduce metal inserts into the elastomeric annular support, in correspondence of the inner wall of the annular support, or to submit the inner wall to surface treatments also intended to reduce the friction thereof.
Such a solution, while reducing the axial friction between the diametrical bearing surface and the support ring, does not allow however for eliminating the previously mentioned drawbacks related to the lack of homogeneity and the wear-induced degradation of the mechanical properties of the elastomeric materials.
Moreover, this manner of operating also causes a reduction of the tangential friction, which should instead be kept high to prevent the outer ring of the bearing from rotating relative to the annular support.
It has therefore been proposed in the past to replace the elastomeric rings by metal rings obtained for instance from an undulated ribbon.
The performance of such rings is generally higher than that of elastomeric rings. The undulated shape actually confers resiliency to said metal rings, the mechanical characteristics thereof are uniform and constant in time, and the metal hardness considerably reduces axial friction with the cylindrical surface of the bearing ring to which they are applied.
Examples of such metal support rings are disclosed in FR 2,789,459, U.S. Pat. No. 5,977,677 and U.S. Pat. No. 6,617,733.
Yet, such solutions are not free from drawbacks, the main of which is due to the fact that the configuration of the ring, obtained for instance from a shaped metal ribbon, has a contact surface with the bearing that is not oriented relative to the rotation direction of the rotating portion.
This configuration has the drawback of considerably reducing also the tangential friction between the ring of the bearing and the support, which friction on the contrary must be sufficiently high to prevent the outer ring of the bearing, to which the support is applied, from rotating relative to the same support.